The season will begin in September, but if you go to Celebration you'll get to see the premiere episodes early.
It will start up where it left Darth Maul & Savage Opress story continues.
There will be a new Yoda model
There is also a new Palpatine model
We are going to see Anakin, Ahsoka and Obi-Wan wearing cloaks.
Bo-Katan will return
Darth Maul & Savage Opress will try to destroy a planet just to get Obi-Wan.
Darth Maul legs gets broken so he gets more natural replacements, Savage losses his arm.
There is no time for Boba Fett this season (he won't appear)
We'll learn more of why Obi-Wan doesn't mind living alone in the desert
We will get to know how Obi-Wan knew "Anakin is the father isn't he?"
There will be apperences of Republic Commando's, at least one new and the return to action Delta Squad!
Tarkin is back! As well as other (classic) officers.
The planet Onderon is going to have a major arc crossing multiple episodes.
Onderon is going to become under the rule of the Separatists.
The Jedi and Republic will fight hard to get the planet back as well, Ahsoka will team up with Lux (Lux's home planet is Onderon) and Hondo will also aid the Republic.
Ahsoka will have the biggest development this far during season 5, her fate will be hinted at
Characters made from earlier seasons will meet their fate (not all but some)
Death Watch will get control of Mandalore by the help of Darth Maul
We will see the armor Boba Fett eventually gets. --- As you can see on the picture it's Boba's armor in a clone wars style, note the fingers are exactly the same style. This picture was posted on the celebration page under a Clone Wars panel article, but was quickly taken down.
Delta Squad will get an entire episode. I don't have much to back that up it's a rumor Dave said it at comic-con but I'm not sure but I know they will make an apperance